Just Instinct
by Elric-Chan
Summary: On Naruto's sixteenth birthday, when he reaches maturity, his fox instincts will temporarily take over. His instinct to mate. And Sasuke won't be able to resist him. SasuNaru


**Disclaimer: Foxes are cute, but they don't belong to me. The same goes for Naru-kun. **

**I think this can be justified as a crazy idea for a story, but it seems entertaining enough. I always wondered if, you know, 'fox rules' would apply to Naruto. Certain ones more than others, I suppose... makes for a good yaoi... **

**Read and review, won't you?**

* * *

"Ohh..." Naruto twisted his head, trying in vain to drive away the stinging crick in his neck. Sasuke turned to look at him, hoisting his knapsack over his shoulder.

"What's wrong, Naruto?"

"Nothing," Naruto said, giving his head a final shake. He stared out at the snow-capped mountains just beyond the Hokage Monument, and shifted his own bag over his shoulder for their journey. "Let's go."

_**JUST INSTINCT**_

Kakashi burst through the doors of Tsunade's office, startling Sakura. The pink-haired kunoichi dropped an armful of papers, and her foot came down on top of the scattered disarray.

"What is it?" she cried. "Is something wrong?"

"Sakura, calm down," Tsunade said, and instantly Sakura relaxed, bending down to retrieve her papers. "Kakashi?"

"Well, it's about Naruto."

At that moment, Iruka and Jiraiya came running in behind him.

"We need Naruto!" Iruka shouted, and Sakura looked up from straightening her pile.

"It's kind of important," Jiraiya cut in. "No, actually, it's extremely important."

"Well, what is it?"

Kakashi cleared his throat. "It's Naruto's birthday tomorrow."

---

Naruto sank to his knees in the powdery snow, and Sasuke lurched forward to catch him. He stared at the blonde's chest, rapidly rising and falling with each ragged breath.

"Naruto... are you sure you're okay?"

"Sure," Naruto coughed, holding a fist in the air. "I'm fine."

Sasuke wasn't completely convinced, and he lifted Naruto's bag from his black-clad shoulder. "I'll take your bag."

"Thanks..." Naruto said weakly, and collapsed face-first into the snow. Sasuke blinked a few times, and crouched down to sling the blonde's limp form over his shoulder as well.

"And I'll take you, I guess."

---

"His birthday?" Sakura stood up, her face incredulous, leaving her papers in a neat stack on the floor.

Tsunade's expression became annoyed. "You came in here like there was a war starting to tell me you need him for his _birthday_?" She snorted. "I expect you need to tell us all of your surprise party plans?"

"No..." Iruka started. "It's not like that at all."

Kakashi sighed. "It's just that... this is his sixteenth birthday."

"And?" Tsunade tapped her foot impatiently.

"Of course you might not know," Iruka cut in. "But there's something that's going to happen to him."

Jiraiya turned and ran back out the door, and for a moment, they all stared at the place where he had been. Then Tsunade cleared her throat.

"Please, the suspense is killing me." She rolled her eyes.

"Iruka, I believe you could explain it better," Kakashi turned to the chunin. "Please do."

"Well..." Iruka certainly looked uncomfortable. In his silence, Sakura slowly bent down and picked up her paper stack. Then Iruka continued, "You know he's got the fox spirit sealed inside him."

"Yes."

"Well, on his sixteenth birthday, when he reaches maturity..." Iruka shifted on his feet. "...his fox instincts will temporarily take over."

"Which means?"

"He'll only be left with those instincts for a little while."

"Instincts? What sort of instincts?"

Iruka made an unintelligible sound, and Kakashi answered for him.

"The instinct to mate."

Everything was silent again. Tsunade glared from Kakashi's unreadable face to Iruka's embarrassed one.

"You're serious?"

"Yes," Kakashi responded. "That's why we need him, to get him away from people for a little while."

Sakura gasped and dropped her papers all over again. _"He's with Sasuke!"_

Kakashi's eyes widened, and Iruka looked completely taken aback.

"That is not good," Kakashi mumbled. "Not good at all."

---

Sasuke stared around at the walls of the cave, his eyes following the dancing patterns of orange light the fire provided. He shifted from his sitting position on the ground, running a hand through his hair. The silence was unnerving.

He looked down at Naruto lying beside him, covered in sweat despite the chill sweeping through the barely-secluded cave. In any normal situation, Naruto's loud voice would be filling the enclosed space; in fact, in a normal situation, they wouldn't have stopped in this cave at all.

He didn't know what was wrong with Naruto, but he hoped that he would get better by tomorrow. Not only did they need to get on with their mission, it was (though Sasuke usually gave no notice to such trivial things) also Naruto's birthday.

He gave a start as the blonde suddenly twitched and sat up, instantly pulling at his jacket. Sasuke turned to help him unzip and slide it off, noticing for the first time just how flushed his tan skin was. Sasuke placed a hand on Naruto's neck; the flesh burned.

"Naruto?" he asked. "Are you... really alright?"

"I'm fine." Naruto replied, sitting up and staring intensely at Sasuke. Even his eyes seemed to be blazing.

---

"So..." Tsunade cleared her throat incoherently while Sakura continued to look horrified. "He'll be like an animal?"

She shuddered, and Sakura blushed, her entire face flooding with crimson. Kakashi sank into a chair, and Iruka went on.

"No, he won't be like an animal," he said, feeling awkward. "He'll just... want to have... sex."

His gaze fell to the floor. He couldn't believe he was talking this way about _Naruto_.

"But... Sasuke will resist." Tsunade said, though she didn't look entirely convinced herself. "Of course he will."

"I'm not so sure," Kakashi said, his face in his palm. "Naruto won't be himself. His desire will be his driving force, and unless Sasuke can contain him, Naruto won't give up."

"But Sasuke _wouldn't_..." Sakura said, her hands shaking. "That's just..."

Kakashi half-shrugged. "It all depends. Think about it, Sakura... what would you do under such intense pressure?"

"But it's... Naruto!" she shouted. "That's like saying it was... me and Ino or something!"

Tsunade looked around at Kakashi. "But surely you can do something?"

"We can," Jiraiya said, reentering the room and pulling a small crystal bottle from his shirt. "With this, we could sedate him and keep him in isolation."

"But we don't know where he is," Iruka finished for him. "And so he'll be uncontrollable."

"Maybe Sasuke can control him..." Sakura put in hopelessly. "He won't do anything with Naruto..."

"We can't guarantee anything." Kakashi said, and Jiraiya replaced the bottle inside his shirt without a word. "It's all up to Sasuke. But Naruto won't take not getting what he's after lightly."

---

Sasuke stared silently at Naruto for a moment, unable to look away from the strong blue glare. It surprised him how fast Naruto seemed to have recovered; sure, he was still burning up, but his voice was clear and he didn't look faint.

Sasuke looked down, still slightly bewildered, as Naruto's fingers closed over his arm.

"Naruto... seriously... you're _sure_ you feel fine? You're acting a little strange..."

Naruto shook his head, and stared out at the snow-drifted night through the small opening crevice of the cave. Sasuke turned and followed his gaze, and three things happened simultaneously.

He felt a quick hand on his neck; he twisted his body as it was pushed down, narrowly avoiding the flames of the fire; and then Naruto's lips were pressed against his.

His onyx eyes widened, and his arms had barely lifted in defense when he felt Naruto's other hand pulling at his pants. His mind reeled as he fought against Naruto, whose tongue was already slipping past his. He locked his arms around the blonde's now bare back, and toppled him over sideways, opposite of the fire.

Naruto's reaction was what shocked him. Rather than fighting back or cowering in shame as Sasuke expected, he fell limp in Sasuke's arms and turned his head to the side, neck exposed. His hands fisted in Sasuke's hair, pulling the raven down again, and his tongue lapped at the pale skin of Sasuke's neck. Sasuke could barely move; he definitely couldn't pull away. Naruto's mysterious strength held him immobile.

"Naruto...!" he choked out at last. "What are you-"

He was cut off as Naruto managed to turn his head, and they locked lips once more. Sasuke's eyes snapped closed as he desperately tried to think of a reason why Naruto would act like this. He slid his hands from behind Naruto's back, bringing them down to the blonde's hips, and instantly Naruto broke away as he moaned, his grip loosening on Sasuke's back.

Sasuke leapt away from him, his heart beating phenomenally fast. Naruto's expression soured, and he sat up, breathing fast as well.

"Please, Sasuke..." Naruto whispered, and Sasuke stared at him with something close to fear shimmering in his eyes. Naruto's own eyes seemed glazed, but they were still intently focused on Sasuke.

"What are you doing?"

"Just please..." He crawled to Sasuke, who sat paralyzed, and leaned up to kiss him again.

---

Tsunade sank down into a chair herself, looking slightly disturbed, and Sakura silently picked up her papers. She drifted out of the room, her eyes still troubled.

"Does Naruto know?" Tsunade asked quietly. Jiraiya shook his head.

"No," he responded. "I wrote him a note explaining it, and put it in his pocket. Hopefully he'll read it... or maybe Sasuke will."

"If..." Tsunade began. "Well, say they _do_ engage in anything... will Naruto remember?"

"No."

Tsunade turned to Kakashi. "He won't? But that will be an awful thing for Sasuke to carry with him."

Kakashi shrugged one shoulder, lifting his face from his hand. "It'll be what it is, I suppose."

"Can Naruto harm him in any way?"

"He can't hurt Sasuke, no," Jiraiya said, and suddenly shifted. "By any normal means anyway... if he's too... rough, that's a different story."

"Where did they go?" Kakashi asked Tsunade. "Is it far from here?"

"They went through the mountains surrounding the Hokage Monument," she said. "They could have stopped anywhere, though."

Kakashi's brow furrowed. He felt almost certain that neither of them would come back pure.

---

"Naru..." Sasuke's eyes drifted shut again, and his breathing became labored as Naruto's lips moved down his neck and over his exposed collarbone. The blond's hands were still busy tugging at his clothes, and Sasuke groaned in spite of himself as those hands finally found their way to the skin of his hips.

Naruto was in his element, kissing and nipping at Sasuke's chest and abdomen. Sasuke didn't want to comply; every part of his brain was screaming in protest, but every part of his body, particularly his lower half, was burning and silently urging Naruto to continue. He numbly raised his hands once again in a futile attempt to push the blonde away, but they fell back to the ground as Naruto's lips left his waist and lowered to his thighs.

He moaned softly, his heart hammering inside his chest. The less rational side of his brain was crying out for Naruto's tongue to move just inches to the left, to the place where he really needed it to be.

"Just a little over... Naruto..." he panted, his voice barely audible. "Please..."

"Sasuke..." Naruto whispered, lifting his head. Sasuke whimpered in protest, but moaned again as Naruto pulled himself onto Sasuke's lap. grinding his hips forward,

"No..." Sasuke was surprised how weak his voice suddenly sounded. "Back... down..."

Naruto shook his head lightly, bringing his lips roughly back to Sasuke's. Sasuke's arms wound around Naruto's back, pulling them as close together as physically possible. He could feel Naruto's heart pounding against his own skin, and for a small instant, he realized how very _wrong_ this was. Naruto couldn't possibly be arousing him like this.

But then his senses slipped away; after all, this was nothing but a warm and tempting body. He was in it for the pleasure, and the fact that this body housed Uzumaki Naruto's soul meant absolutely nothing.

"There... that's the right way..." Naruto crooned, and bright lights flared before Sasuke's closed eyes as he felt pressure descend over his burning arousal. For a moment he couldn't tell if it was Naruto's hand; it certainly wasn't his mouth, as that was still devouring his own. He soon understood that it was not Naruto's hand at all (for they were both securely fisted in his own ebony hair), but that it was the blonde's entrance that he had slipped into.

He cried out in anger, embarrassment, and lust, as his hands acted of their own accord and guided Naruto's movements. Naruto's moans were loud in his ear, his heart was thrashing inside him, and what was left of his sanity made a plea.

_Don't think about Naruto..._ his mind begged. _Don't think about Naruto..._

Without conscious effort, he pushed Naruto back so that the blonde was sprawled beneath him, close enough to the fire to make their bodies seem even hotter. Naruto's hands refused to loosen their grip on Sasuke's hair, even as he arched his back and cried out with each of Sasuke's thrusts.

"Sasuke..." And suddenly Naruto snarled, a feral noise that ripped from his throat, so very unlike him. But Sasuke lunged forward and grasped him behind the head as he he felt release; in spite of this, his head felt clear.

_It had to be the fox._

Only the fox would have caused him to act like this, Sasuke realized as he pulled away from Naruto's still shivering form, but the reason for that he didn't know. He pulled his body back against the cave wall, watching for a few tense minutes as Naruto's breathing slowed and his own did as well, and he wrapped his arms around himself as Naruto fell into a deep sleep.

---

"Sakura?"

Kakashi sidled slowly into the darkened room, where Sakura sat quietly on the windowsill.

"Sensei?" Her face was turned away from him, her voice calm, her eyes focused on the snow-capped peak of a distant mountain. "Is Tsunade-san still here?"

"She's in her office."

"You don't think Sasuke will really do what you said her would, do you?" she asked idly. "I mean, you can't be certain."

"No, I can't."

Sakura turned to face him and stood up. "I'm sure he'll be fine."

---

Sasuke breathed deep; once, twice, three times in the cold air outside the cave. He scooped a handful of snow into his palms and rubbed it vigorously into his face, and stared out at the sky. He turned back to the cave, back to where Naruto lay beside the fire, his naked body covered with a blanket. He had cleaned Naruto's clothes in the snow, and dried them by the fire. His own had not been dirtied, just merely ruffled from Naruto's prying hands.

Naruto was sleeping soundly, his skin still flushed a dull crimson, his hair sweaty and disheveled. Sasuke looked down once again at the note he had found in one of Naruto's pockets.

_To whomever may be reading this, there is something you must know. Because of the spirit of the Nine-Tailed Fox that is sealed inside Naruto's body, he will be overcome with the instinct to mate when he reaches maturity on his sixteenth birthday. This will last for only a short period of time, but this is fair warning. Naruto will be uncontrollable. He will carry nothing but the desire to have sex, and he will see to it that that desire is fulfilled, using force if necessary. There are plans to remove him to isolation before this can happen, but if he should be out of contact when this event arises, anyone in company with him should take caution. Naruto will not remember any of his actions during this time._

Sasuke frowned. He had an answer, but he felt that it wasn't quite the one he'd been looking for.

So Naruto had reached maturity, and for that, the evil fox had sought to make him behave like a mindless animal. To give up his virginity for reasons beyond his control. Sasuke looked back down at the letter.

_Naruto will not remember any of his actions during this time._

So he would forever have to carry the memory of having sex with Naruto, and yet Naruto would be none the wiser once he returned to his normal state of mind. It was an injustice, Sasuke thought furiously. Naruto couldn't go around thinking he was pure when he wasn't.

_And yet,_ Sasuke argued with himself. _Naruto _**is** _pure. His body may be tainted, but that was not his decision. You made the distinction between body and soul very clear when it was Naruto's tongue in your mouth._

He was suddenly furious with himself. How could he have let himself lose control like that?

Naruto stirred, and Sasuke looked over at him, surprised to feel his own expression soften as his gaze fell on the blonde. He liked to think that Naruto could believe he was innocent, and he himself could believe it too, maybe.

Not that anyone who knew Naruto's secret would be fooled. Anyone, especially whomever it was that had written the note, that knew where Naruto had been and who had been with, would not have any doubt that they had had sex.

Sasuke sighed, and the sound seemed to trigger Naruto's eyes to open. Sasuke stared curiously into them, but they were still resolutely glazed. He remained still as Naruto sat up, a smile on his face, the blanket falling to reveal his tanned chest. No sooner than he had started to move forward did Sasuke meet him halfway, pulling him back to the other stone wall to save the trouble of having the blanket fall all the way.

Naruto's lips met his eagerly, and Sasuke simply sighed and kissed him back. It was all he could do.

---

Sasuke awoke to a ray of sun warming his face, and he opened his eyes, blinking rapidly. He eyes slid over to the fire, which was down to just a few flickering embers. They fell on the illuminated cave walls, the two bags sitting against one of the walls, and finally on Naruto. He looked down to see that their fingertips were inches apart, as though they had fallen asleep while still wrapped in an embrace. Emotion bubbled up inside of him, and he sat up, quickly drawing Naruto's still-naked form to his chest.

"Good morning, Sasuke."

Sasuke promptly yelped and dropped Naruto back down, recoiling against the wall as Naruto rubbed his neck.

"Ow, Sasuke, that hurt..." Naruto complained, while Sasuke watched him with shaking eyes. "If you're going to hug me, don't drop me."

"Naruto..." Sasuke managed, his voice strangled.

"Mm?" Naruto sat up, turning his blue eyes on Sasuke.

"You..."

Naruto's eyes seemed to melt, and a gentle smile crept onto his lips as he turned to look out at the sun.

"I'm sorry about last night... Sasuke."

Sasuke's hands curled into fists at his sides, and he drew in his breath sharply. Naruto turned to him, still smiling softly.

"You thought I wouldn't remember?" He asked, and shook his head slowly. "Of course I do. I can't tell you how horrible it was being trapped behind my own eyes, not even able to control my own body."

Sasuke stared at him in horror. Naruto chuckled at his incredulity and went on.

"Not that it... was horrible," he said, a blush staining his cheeks. "Just to feel like I was watching from the outside, only to know that it was me I was watching. The fox was laughing at me."

"You... you remember?" Sasuke whispered, almost inaudibly. Naruto nodded.

"I have to admit, I could feel my senses coming back late into the night, but it... felt so nice to kiss you that I just gave in."

Sasuke flushed from his neck to his hairline, and Naruto looked down at his clothes lying in a pile by the fire.

"Thanks for cleaning my clothes," he told Sasuke, pulling them on with his back to the raven. He stopped as he made to zip up his jacket, and turned to Sasuke.

"Was it really so hard to resist?"

Sasuke swallowed hard, his face still burning. "The note said it would be."

Naruto's expression became puzzled. "Note? What note?"

Sasuke raised a shaky finger and pointed at the scrap of paper lying by their bags. Naruto reached out an arm and picked it up, his eyes scanning it quickly. He barked a short laugh as he threw it down again.

"That's Jiraiya's handwriting," he scoffed. "Like the pervy sage knows anything about it."

"So the note was wrong?"

"The majority of it was true, I guess. But I definitely remember."

"Would you have, well... 'used force'?"

"The fox probably would have."

Sasuke looked down at the ground again. In a flash, Naruto was at his side, a warm hand held to his face.

"Don't worry about it, Sasuke," Naruto told him quietly, azure scorching into obsidian. "It's not your fault."

"It... it doesn't bother you?" Sasuke asked, having trouble finding his voice.

"To be honest, no. But what an embarrassing first time that was."

"That was your-" Sasuke shook his head. "No, sorry, that was foolish..."

"Of course it was my first time," Naruto replied, unswayed. "And yours?"

"Yeah."

Naruto leaned in closer, and suddenly Sasuke realized that the blonde's legs were straddling his. He paused with his lips centimeters from Sasuke's, their eyes still locked.

"You don't mind?"

Sasuke shook his head. He'd gotten quite used to the feel of Naruto's lips on his.

Naruto's lips turned up in a smile before they pressed together with Sasuke's. He made a noise of surprise as he felt Sasuke's arms tighten around his back, pulling him deeper into the kiss, but he obliged. Only when things began to get a bit feverish, moreover when his jacket was once again being pulled from his frame, did he pull away.

"No, Sasuke..." he said breathlessly. "We should... get back to Konoha..."

"Our mission..."

"Our mission is over," Naruto answered. "We just need to go home."

"Of course," Sasuke murmured as Naruto climbed off of his lap. "Yeah..."

They set about gathering up the blankets and bags, and Sasuke blew out what remained of the fire. Naruto pulled his sandals back on, and hoisted his bag over his shoulder as Sasuke did the same. He crawled out of the cave, closely followed by Sasuke, and for a minute, they both stood looking at the sun.

"It's a beautiful day."

"Yeah..." Sasuke looked sideways at Naruto. "Happy birthday."

Naruto grinned over at him. "Quite a price to pay for turning sixteen, yes?"

Sasuke didn't answer, but instead took Naruto's hand in his own and pulled him down the path they had taken up the mountain. The blonde smiled over at him.

"Would you have told me, if I really hadn't remembered anything at all?"

Sasuke was silent for a minute. "Probably."

"Are you going to tell everyone?"

Sasuke shot a crazed look at him. "Of course not!"

Naruto laughed. "I mean the people who knew about it anyway. I for one am going to throw it in the pervy old sage's face that I remembered what happened."

"If they ask, I will."

"I guess they expect it, don't they?" Naruto grinned again. "There's no way you would have come back without having given in to me."

Sasuke scowled at him. "Whatever."

"Did you enjoy yourself?" Naruto asked, clearly taunting Sasuke now. "Was I good?"

With a growl, Sasuke threw Naruto against the nearest rock face. The smile vanished from Naruto's lips, and he raised his hands in apology.

"I'm sorry," he said. "I was just kidding."

With a sigh, Sasuke released Naruto. "It's hard enough for me to think about."

"Oh?"

"I didn't enjoy myself, if you must know." Sasuke told him. "I was too busy trying to figure out what was going on, and too busy concentrating on not thinking that I was have sex with you."

"But you did like it?" Naruto's voice sounded hurt. Sasuke looked wildly back at him, not knowing what to say.

"I... yes, I did... eventually."

"I guess it was because your body took over."

"Yes." Sasuke frowned. He'd had all night to consider things, but Naruto had broken that composure when he'd woken up and distracted Sasuke.

"Well..." Naruto threw Sasuke off balance as he pulled him into another kiss, this one fiercely more passionate than before, and Sasuke found himself now the one backed into the rock wall.

As before, he gave up and secured Naruto in his arms, but just as he did, the blonde pulled away. A triumphant smirk was settled on his lips.

"I told you that you couldn't resist me, even when it _is_ me and not the fox."

Sasuke closed his eyes and rested the side of his head on the wall, his hands lowering from Naruto's back to the rough rock, and suddenly his fingers discovered a crevice.

He lifted his head, and looked around Naruto's shoulder to see another opening in the rock wall, one that looked just large enough for two bodies to fit comfortably in.

What were the odds?

Without a second thought, he pulled Naruto into the new cave, roughly bringing their lips together again.

They would have all day to return to Konoha.

_**OWARI**_

* * *

**A/N: Whew, this could be one of my new faves. I have the lingering feeling that there's something I could have done better, but I'm pleased with this all the same. And I would absolutely love it if you would be lovely and review for me.**


End file.
